The invention relates to novel liquid crystal compounds, and more particularly to novel ferroelectric liquid crystal compounds showing tri-stable phases, particularly suitable for electrooptical image or display elements.
As electrooptical devices utilizing nematic liquid crystals, there have been proposed and actually used various devices of dynamic scattering mode (DSM), twisted nematic (TN) mode, guest-host (GH) mode and satellite tracking network (STN) mode. The devices of such nematic liquid crystals, however, are generally disadvantageous in the slow speed of response in the order of several m sec- several decades of m sec so that there are considerable limitations in the applied uses thereof. It is a reason of such slow speed of response that the torque to drive molecules of the liquid crystal compounds is fundamentally based on the dielectric constant anisotropy so that the torque force substantially dominates the property or performance thereof.
Thus, the ferroelectric liquid crystals were developed by Meyer et al (Le Journal de Physique, 36 (1975)-L-69). In JP-A No. 63-307837, improved ferroelectric liquid crystals showing bi-stable phases are disclosed. Some electrooptical devices utilizing such ferromagnetic liquid crystals and consequently having higher speed of response were proposed. For instance, JP-A No. 56-107216 discloses a device in which two molecular orientations, which are made parallel to a wall surface by releasing the twisted structure thereof owing to the force of the wall surface, may be varied depending on the polarity of the impressed electric field, which is theoretically based on the presence of a liquid crystal compound showing ideal bi-stable phase as shown in wave shapes of response to the electric field, FIG. 5.
In fact, however, such ideal compounds have not yet been found. The actually found liquid crystal shows the wave shapes of response to the electric field as shown in FIG. 6. When using such liquid crystal e.g. in a photoswitching circuit, as the impressed voltage is changed from the .crclbar. side to the .sym. side, the transmission factor (%) is gradually changed so that the simple ON-OFF change of the impressed voltage can not sufficiently attain the purpose.
Furthermore, so far as the liquid crystals of bi-stable phases having been synthesized until now are concerned, it is impossible to form a monodomain state, which is a perfect molecular orientation, due to an inevitable failure called discrimination or a disorder of the molecular orientation called twist, in the S.sub.A phase where no electric field is applied. Thus, it is difficult to realize such perfect two states orientation in a larger area.
Furthermore, when dynamically driving, inevitably the contrast is lowered and the visual field angle is narrowed due to the low threshold of the voltage causing a change of brightness in a specific range. Since the liquid crystals of the bi-stable phase having been synthesized until now show not the hysteresis as shown in FIG. 5 but the hysteresis as shown in FIG. 6 so as not to have the memory effect. In order to hold the stable Sc phase for the liquid crystals, it is necessary to continuously impress the voltage V.sub.3 in FIG. 6 or continuously apply a high frequency, either of which causes a considerably much amount of energy loss.